Optical recording medium, manufacturing method for optical recording medium, and reproducing method for optical recording medium

ABSTRACT

An optical recording medium includes a main-information area in which a metal reflection film is formed on a substrate where a row of pits is formed as main data, and a sub-information area in which is recorded medium identification information, which is used to identify the optical recording medium individually, by removing the metal reflection film partially and forming a plurality of reflection-film removed areas. Information is reproduced by irradiating the metal reflection film with a beam of light. In the sub-information area, a row of pits or a guide groove is formed on the substrate, and a track pitch of the row of pits or the guide groove is at least 0.24 μm wide and at most 0.45 μm wide.

This is a divisional application of Ser. No. 12/194,276, filed Aug. 19,2008, which is a divisional application of Ser. No. 10/509,745, filedSep. 30, 2004, which is the National Stage of International ApplicationNo. PCT/JP2004/003459, filed Mar. 16, 2004.

TECHNICAL FIELD

The present invention relates to an optical recording medium,particularly, an optical disk which is shaped like a circular plate andis used to reproduce information.

BACKGROUND ART

As a conventional optical recording medium, for example, there is anoptical disk, such as a CD-ROM and a DVD-ROM. In such an optical disk,an uneven row of pits is formed on a transparent substrate which is madeof polycarbonate or the like. On the substrate, a metal reflection filmis formed which is made of Al or the like. From a side of a surfaceopposite to a surface on which this metal reflection film is formed, abeam of light is applied to the metal reflection film which is aninformation recording surface. Thereby, information is reproduced.

Such an optical recording medium has been widely used in whichinformation is recorded and reproduced by applying a beam of light.Thus, expectations have become greater of heightening its recordingdensity from now on. In recent years, a variety of optical disks hasbeen developed which can reproduce large-capacity audio-visual data ordigital data. For example, research and development for a high-densityROM optical disk is now going on, in which a density of an optical diskwhich has a diameter of 12 centimeters is expected to become higher to astorage capacity of 23.3 to 30 gigabits.

On the other hand, a DVD ROM recording medium is provided with asecurity technique, specifically, a technique of preventing someone fromillicitly using and copying recorded information or from doing such anact. As that security technique, a BCA (or burst cutting area) area isprovided where medium identification information, which is used toidentify each recording medium individually, is overwritten in abar-code pattern. In this BCA area, when an optical recording medium ismanufactured, medium identification information which differs for eachoptical recording medium is recorded, and if necessary, a key ofcryptograph or a key of decoding is recorded.

For example, Japanese Patent Laid-Open No. 10-233019 specificationdiscloses that a metal reflection film of an optical disk on which a rowof pits is formed as main data is partially removed by laser trimming,and modulated data is recorded individually. Thus, medium identificationinformation is recorded which is used to protect against illicitly usingand copying, or such an act.

However, in order to heighten the above described density, a pitchbetween tracks has to be narrowed, or a shortest pit of a row of pitsneeds to be shortened. Besides, with respect to a high-density opticaldisk, at least 23.3 GB data is recorded on a 12 cm-diameter opticaldisk. Therefore, it has been determined that if on a substrate used forsuch an optical disk, a metal reflection film is formed which is made ofan Al alloy material having a film thickness of 50 to 70 nm so that itcan be used in a DVD ROM optical disk, that deteriorates quality of areproduced signal.

This is because a metal reflection film seems to be difficult to form ata bottom of a minute pit about 0.2 μm long. Thus, the shorter a pitbecomes, the deeper and the smaller it tends to be. Accordingly, as ametal reflection film for the above described high-density ROM opticaldisk, a metal reflection film which is used in a DVD ROM optical diskcould not be used as it is.

In addition, when a DVD ROM optical disk is manufactured, mediumidentification information is recorded, using amedium-identification-information recording apparatus which is providedwith a YAG (yttrium aluminum garnet) laser. However, even if the mediumidentification information is recorded in a bar-code pattern using thismedium-identification-information recording apparatus, on an area wherepits are not formed in a high-density ROM optical disk or on a row ofpits which is recorded at a track pitch of 0.74 μm, which is the same asin the DVD ROM optical disk, then a pattern could not be formed. Or,reproduction noise of the medium identification information becamelouder, and thereby, an adequate defocus margin could not be secured.

This is because in a high-density ROM optical disk, a metal reflectionfilm is thinner than that of a DVD ROM optical disk. Or, material of ametal reflection film in use is different, and thus, heat capacitynecessary until the metal reflection film reaches its melting point islargely different. Accordingly, a conventionalmedium-identification-information recording apparatus provided with aYAG could not be used as it is when a high-density ROM optical disk ismanufactured.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical recordingmedium in which data can be recorded more densely than in a DVD ROMoptical disk, and by using a conventionalmedium-identification-information recording apparatus, mediumidentification information can be recorded so that an adequate defocusmargin can be secured.

An optical recording medium according to an aspect of the presentinvention includes: a main-information area in which a metal reflectionfilm is formed on a substrate where a row of pits is formed as maindata; and a sub-information area in which is recorded mediumidentification information, which is used to identify the opticalrecording medium individually, by removing the metal reflection filmpartially and forming a plurality of reflection-film removed areas.Information is reproduced by irradiating the metal reflection film witha beam of light, and in the sub-information area, a row of pits or aguide groove is formed on the substrate, and a track pitch of the row ofpits or the guide groove is at least 0.24 μm wide and at most 0.45 μmwide.

In this optical recording medium, a row of pits or a guide groove isformed in the sub-information area on the substrate, and the track pitchof the row of pits or the guide groove is set to be at least 0.24 μmwide and at most 0.45 μm wide. Therefore, by using a beam of light forreproduction having a shorter wavelength and an optical system having ahigher numerical aperture, data can be recorded at a higher density thanin a DVD ROM optical disk. In addition, even though thermal conductivityor a melting point, which is an intrinsic value of the metal reflectionfilm is different, by using a conventionalmedium-identification-information recording apparatus, mediumidentification information can be recorded so that an adequate defocusmargin can be secured.

A manufacturing method for an optical recording medium according toanother aspect of the present invention, includes: a first step ofpreparing a substrate on which a row of pits is formed as main data in amain-information area, and a row of pits or a guide groove whose trackpitch is at least 0.24 μm wide and at most 0.45 μm wide is formed in asub-information area; a second step of forming a metal reflection filmon the substrate; a third step of forming a resin layer on the metalreflection film; and a fourth step of recording medium identificationinformation which is used to identify the optical recording mediumindividually by partially removing the metal reflection film in thesub-information area and forming a plurality of reflection-film removedareas.

By this manufacturing method for an optical recording medium, the row ofpits or the guide groove is formed in the sub-information area on thesubstrate, and the track pitch of the row of pits or the guide groove isset to be at least 0.24 μm wide and at most 0.45 μm wide. Therefore, byusing a beam of light for reproduction having a shorter wavelength andan optical system having a higher numerical aperture, data can berecorded at a higher density than in a DVD ROM optical disk. Inaddition, even though the thermal conductivity or melting point, whichis the intrinsic value of the metal reflection film, is different, byusing a conventional medium-identification-information recordingapparatus, medium identification information can be recorded so that anadequate defocus margin can be secured.

A reproducing method for an optical recording medium according to stillanother aspect of the present invention, in which the optical recordingmedium includes a main-information area in which a metal reflection filmis formed on a substrate where a row of pits is formed as main data, anda sub-information area in which a row of pits or a guide groove whosetrack pitch is at least 0.24 μm wide and at most 0.45 μm wide is formedon the substrate, includes medium identification information beingrecorded which is used to identify the optical recording mediumindividually by removing the metal reflection film partially and forminga plurality of reflection-film removed areas, and information isreproduced by irradiating the metal reflection film of the opticalrecording medium with a beam of light.

By this reproducing method for an optical recording medium, informationis reproduced by applying a beam of light to the metal reflection filmof the optical recording medium which includes a sub-information areawhere the row of pits or the guide groove is formed in thesub-information area on the substrate and the track pitch of the row ofpits or the guide groove is set to be at least 0.24 μm wide and at most0.45 μm wide. Therefore, by using a beam of light for reproductionhaving a shorter wavelength and an optical system having a highernumerical aperture, a good-quality signal can be obtained by reproducingdata which has been recorded at a higher density than in a DVD ROMoptical disk. In addition, even though the thermal conductivity ormelting point, which is the intrinsic value of the metal reflectionfilm, is different, by using a conventionalmedium-identification-information recording apparatus, the mediumidentification information which has been recorded at an adequatedefocus margin can be steadily reproduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation, showing a measurement result of ajitter value which corresponds to a depth of a pit.

FIG. 2 is a graphical representation, showing a measurement result of ajitter value which corresponds to film thickness of a metal reflectionfilm which is made of an AgPdCu alloy.

FIG. 3 is a graphical representation, showing a measurement result of ajitter value which corresponds to film thickness of a metal reflectionfilm which is made of an Al alloy.

FIG. 4 is a sectional view of an optical disk in which a metalreflection film, which is made of an AgPdCu alloy and has a filmthickness of 100 nm, is formed on a substrate where pits are formed.

FIG. 5 is a graphical representation, showing a measurement result of areflectance ratio which corresponds to film thickness of a metalreflection film which is made of an AgPdCu alloy.

FIG. 6 is a graphical representation, showing a measurement result of areflectance ratio which corresponds to film thickness of a metalreflection film which is made of an Al alloy.

FIG. 7 is a top view of an optical disk, showing an example of itsmain-information area and sub-information area.

FIG. 8 is a block diagram, showing a configuration of amedium-identification-information recording apparatus which recordsmedium identification information in a BCA area.

FIG. 9 is a sectional view of an optical disk in which a metalreflection film is formed on a substrate where pits are formed, and inaddition, a resin layer is formed on the metal reflection film.

FIG. 10 is a graphical representation, showing a measurement result of adefocus margin of BCA recording power which corresponds to a track pitchof a row of pits which is formed in an optical disk that includes a 50nm thick metal reflection film which is made of an AgPdCu alloy.

FIG. 11 is a graphical representation, showing a measurement result of adefocus margin of BCA recording power which corresponds to a track pitchof a row of pits which is formed in an optical disk that includes an Alreflection film whose film thickness is 30 nm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a ROM optical disk will be described as an example of anoptical disk according to an embodiment of the present invention.Herein, an optical recording medium which is applied according to thepresent invention is not limited especially to this example. The presentinvention can also be applied to various optical recording mediums whoseinformation recording layer has, for example, a minute unevenness, suchas an optical magnetic disk and a phase-change disk.

The ROM optical disk includes: a main-information area in which a metalreflection film is formed on a substrate where an uneven row of pits isformed as main data; and a sub-information area in which mediumidentification information is recorded, which is used to identify theoptical disk individually, by removing the metal reflection filmpartially and forming a plurality of reflection-film removed areas. Inthis optical disk, information is reproduced by irradiating the metalreflection film with a beam of light.

Generally, in order to heighten density of a ROM optical disk, a pitchbetween tracks has to be narrowed, and a shortest pit length (or ashortest mark length) needs to be extremely shortened. However, if atrack pitch becomes too narrow, cross talk becomes greater in anRF-signal characteristic. This hinders securing an adequate systemmargin. If the shortest pit length becomes too short, then resolution ofa reproduced signal lowers, thereby worsening a jitter value of areproduced signal.

Therefore, an examination is repeatedly made of a most suitable trackpitch, using an information reproducing apparatus in which a wavelengthλ of a light source of a beam of light for reproduction is 405nm and anumerical aperture NA of an objective lens is 0.85. As a result of suchan examination, the following measurement result is obtained. Thispresents the fact that if a track pitch is at least 0.24 μm wide, across-talk signal can be practically neglected, compared with a mainsignal.

Track Pitch Jitter (μm) Value (%) 0.20 7.6 0.22 7.0 0.24 6.5 0.26 5.60.28 5.4

In addition, a most suitable shortest pit length is examined, using theabove described information reproducing apparatus. As a result of astudy of a resolution necessary for obtaining a desirable reproductionsignal, a measurement result is obtained as follows. It has turned outthat if a length of the shortest pit is at least 0.12 μm long,resolution of a reproduced signal can be adequately secured.

Shortest-Pit Jitter Length (μm) Value (%) 0.10 8.2 0.11 6.8 0.12 6.50.13 5.4 0.14 5.3

Herein, in consideration of various margins of an optical disk or adrive, a jitter value which shows characteristics of an optical diskneeds to be at most 6.5%.

Herein, information on a 12 cm-diameter optical disk is reproduced,using the information reproducing apparatus. In order to set a storagecapacity of the optical disk to at least 23.3 GB, a relationalexpression (shortest pit length)×(track pitch)≦0.0512 μm² has to besatisfied. For example, if the recording capacity is 23.3 GB and theshortest pit length is 0.12 μm, an upper limit of the track pitch isabout 0.43 μm. In the same way, if the recording capacity is 23.3 GB andthe shortest pit length is 0.24 μm, an upper limit of the track pitch isabout 0.21 μm.

Next, a manufacturing method will be described for a 12 cm-diameteroptical disk which has a recording capacity of at least 23.3 GB. Asdescribed above, in order to create a 12 cm-diameter optical disk whichhas a recording capacity of at least 23.3 GB, a substrate has to be usedwhose track pitch is at least 0.24 μm wide and at most 0.43 μm wide, andits shortest pit length is at least 0.12 μm long and at most 0.21 μmlong.

For example, in order to create a 12 cm-diameter optical disk which hasa recording capacity of 25 GB, first, a substrate is prepared where arow of pits is formed which has a shortest pit length of 0.149 μm and atrack pitch of 0.32 μm. As this substrate, for example, a substrate madeof polycarbonate can be used which is created by an injection moldingmachine.

Next, a metal reflection film is formed on this substrate, using a filmformation apparatus. As the film formation apparatus, one which can forma metal reflection film uniformly, such as a magnetron sputteringapparatus and a vapor depositing apparatus, can be used. For example,using a magnetron sputtering apparatus, a time for film formation can bevaried, thereby controlling a film thickness of the metal reflectionfilm. Herein, the material, film thickness, or the like, of the metalreflection film will be described later.

Next, the optical disk is placed on a spin coater, with the metalreflection film facing upwardly. Then, a resin to be hardened byultraviolet rays is dripped, and on top of the resin, an 88 μm-thicktransparent sheet which is made of polycarbonate is placed. In thisstate, the resin is irradiated with ultraviolet rays while the opticaldisk is being rotated by the spin coater. At this time, a rotationalspeed of the spin coater is controlled, so that a thickness of the resinafter it has hardened becomes 12 μm. As a result, a transparent resinlayer which has a film thickness of 100 μm is formed on the metalreflection film. For example, an acrylic resin can be used as thisresin.

In such a way as described above, the metal reflection film was formedon the substrate where a row of pits was formed which had a shortest pitlength of 0.149 μm and a track pitch of 0.32 μm. On top of it, a resinlayer which had a film thickness of 100 μm is formed, and consequently,an optical disk is manufactured.

Next, with respect to the optical disk which was manufactured asdescribed above, a study was made of a depth of a pit which correspondsto quality of a reproduced signal, material and film thickness of themetal reflection film, and the like. Specifically, the manufacturedoptical disk was set in the above described information reproducingapparatus. Then, this information reproducing apparatus allowed a beamof light to be incident upon the metal reflection film through the 100μm-thick resin layer. Thereby, a reproduced signal was obtained from theoptical disk, and then, this signal was assessed.

First, an examination was made as to how much quality of a reproducedsignal depended upon depth of a pit. In the optical disk which wasmanufactured as described above, jitter values were measured whichshowed dispersion of reproduced signals when the depth of a pit varied.FIG. 1 is a graphical representation, showing a measurement result ofthe value of a jitter which corresponds to the depth of a pit. Itshorizontal axis is the depth (nm) of a pit and the vertical axis is thevalue (%) of a jitter. In FIG. 1, as the metal reflection film, one wasused which was made of an Al alloy with a purity of 99 wt % and had afilm thickness of 25 nm. However, even when one was used which was madeof an Ag98Pd1Cu1 (wt %) (hereinafter, referred to as the AgPdCu alloy),the same result as the following was obtained.

Generally, in order to secure an adequate system margin, the value of ajitter has to be at most 6.5%. In FIG. 1, it could be seen that if thedepth of a pit is set to at least 44 nm and at most 88 nm, the value ofa jitter would be at most 6.5%. Herein, a refractive index n of thecreated resin layer was 1.53, and a wavelength λ of the beam of lightwas 405 nm. Therefore, taking the above described measurement resultinto account, one can see that a depth D of a pit at which a desirablereproduction signal would be obtained is at least λ/(6×n), and at most λ/(3×n).

This seems to be for the following reason. Specifically, the depth of apit affects amplitude of a reproduced signal, and in an opticalcalculation, when the depth of a pit is λ/(4×n), the amplitude becomesmaximum. If a refractive index n of the resin layer is 1.53 and thewavelength λ of the beam of light is 405 nm, the amplitude becomesmaximum when the pit depth is about 66 nm. But, even if the amplitudebecomes a little smaller, the jitter value of a reproduced signal isalmost unchanged. However, if the pit depth is below λ/(6×n), or if thepit depth is above λ/(3×n), then an adequate signal-to-noise ratio(hereinafter, referred to as an S/N ratio) cannot be obtained, therebyworsening the jitter value of a reproduced signal.

Next, a study was made of a suitable film thickness of a metalreflection film. First, a substrate was prepared in which the depth of apit is λ/(4×n). As the metal reflection film, two kinds were used whichwere a metal reflection film which was made of an AgPdCu alloy and ametal reflection film which was made of an Al alloy with a purity of 99wt %. Then, value of a jitter was measured when their film thickness wasvaried. FIG. 2 is a graphical representation, showing a measurementresult of a jitter value which corresponds to a film thickness of themetal reflection film which is made of the AgPdCu alloy. FIG. 3 is agraphical representation, showing a measurement result of a jitter valuewhich corresponds to a film thickness of the metal reflection film whichis made of the Al alloy. In each figure, the horizontal axis is the filmthickness (nm) of the metal reflection film, and the vertical axis isthe value (%) of a jitter.

As can be seen in FIG. 2, in a case of the metal reflection film of theAgPdCu alloy, if its film thickness was at least 25 nm and at most 75nm, the value of a jitter became at most 6.5%. On the other hand, asshown in FIG. 3, in a case of the metal reflection film of the Al alloy,if its film thickness was at least 15 nm and at most 40 nm, the value ofa jitter became at most 6.5%. Herein, material of a metal reflectionfilm is not limited especially to those in the examples. Anothermaterial may also be used, as long as it has a high reflectance ratioand can be uniformly formed on a substrate by a film formationapparatus. In addition, in order to enhance its corrosion resistance, arare-earth metallic element such as Nd, or a transition metallic elementsuch as Ti and Cr, may also be added a little to an Ag or Alreflection-film material.

Next, a reflectance ratio of a metal reflection film was examined. Thethinner a metal reflection film becomes, the smaller a quantity ofreflected light will be. Then, when the quantity of reflected lightbecomes smaller, in proportion to that, a medium noise also lowers. Thiskeeps the S/N ratio unchanged. On the other hand, system noise or lasernoise does not depend upon the quantity of reflected light. If thesystem noise or the laser noise is far lower than medium noise so thatit can be neglected, then it will not affect quality of a reproducedsignal, even though the quantity of reflected light becomes smaller.

However, if the quantity of reflected light becomes smaller, and thesystem noise or the laser noise reaches the same level as the mediumnoise, then the quality of a reproduced signal will deteriorate when thequantity of reflected light decreases. Besides, if the metal reflectionfilm is made of a different material even though it has the same filmthickness, that will change its reflectance ratio, and thereby, willchange the film thickness at which signal quality worsens. In addition,if the metal reflection film becomes thicker, a reproduced signal willbecome worse. For example, in a magnetron sputtering apparatus, metallicatoms on a target which have been sputtered by Ar ions come flying ontoa substrate, so that a metal reflection film is formed. A size of thesemetallic atoms also depends upon structure of a film formationapparatus, or conditions of film formation. But such a film tends to bedifficult to form at a bottom of the shortest pit.

FIG. 4 is a sectional view of an optical disk in which a metalreflection film, which is made of an AgPdCu alloy and has a filmthickness of 100 nm, is formed on a substrate where pits are formed. Asshown in FIG. 4, a shortest pit 11 and a long pit 12, which is longerthan the shortest pit 11, are formed on a substrate 1. In this case, ata bottom of the shortest pit 11, a metal reflection film 2 is moredifficult to form than at a bottom of the long pit 12. Therefore, theshortest pit 11 after the metal reflection film 2 has been formedbecomes smaller, and at the same time, deeper than it was on thesubstrate 1.

If one anticipates this phenomenon, and thus, makes a recording powergreater so that the shortest pit 11 can be greater, then signal qualityof the shortest pit 11 will improve. However, when recording powerbecomes greater, the long pit 12 will be wider. This makes cross talkgreater which comes from adjacent tracks, thus worsening the value of ajitter. In consideration of factors which can worsen the signal qualityof both kinds of films, substrates were formed which were suitable formetal reflection films of an Al alloy and an AgPdCu alloy. As a result,a maximum film thickness of the Al-alloy metal reflection film at whichthe value of a jitter was prevented from worsening was 40 nm, and amaximum film thickness of the AgPdCu-alloy metal reflection film atwhich the value of a jitter was prevented from worsening was 70 nm.

Based on this study, a reflectance ratio was measured which correspondedto the film thickness of each of the AgPdCu-alloy metal reflection filmwhich is shown in FIG. 2 and the Al-alloy metal reflection film which isshown in FIG. 3. FIG. 5 is a graphical representation, showing ameasurement result of the reflectance ratio which corresponds to thefilm thickness of a metal reflection film which is made of an AgPdCualloy. FIG. 6 is a graphical representation, showing a measurementresult of the reflectance ratio which corresponds to the film thicknessof a metal reflection film which is made of an Al alloy. In each ofFIGS. 5 and 6, the horizontal axis is the film thickness (nm) of themetal reflection film, and the vertical axis is the reflectance ratio(%). Herein, the refractive index n of the resin layer which was usedfor measurement is 1.53, and the wavelength λ of the beam of light is405 nm.

As can be seen in FIG. 5, in the case of the AgPdCu-alloy metalreflection film, the reflectance ratio which corresponded to a filmthickness of 25 nm to 70 nm at which a desirable jitter value wasobtained was 35% to 70%. In the case of the Al-alloy metal reflectionfilm in FIG. 6, the reflectance ratio which corresponded to a filmthickness of 15 nm to 40 nm at which a desirable jitter value wasobtained was 35% to 70%. As a result, for each film, the reflectanceratio of the metal reflection film at which the quality of a reproducedsignal could be guaranteed was at least 35% and at most 70%.

Next, in order to obtain a reproduction signal which has a desirablejitter value in such a way as described above, an uneven row of pits isformed as main data in a main-information area of an optical disk. Adetailed description will be given of medium identification informationwhich is formed in a sub-information area of the optical disk. FIG. 7 isa top view of an optical disk, showing an example of itsmain-information area and sub-information area.

In the example shown in FIG. 7, a main-information area 21 (which is ahatching part in this figure) is set in an outer circular part on theoptical disk. In a ring-shaped part inside of the outer circular part, aBCA area 22 (which is an area between two circles shown by broken linesin the figure) is set which is a sub-information area. In the BCA area22, medium identification information 23 is recorded in a bar-codepattern. A transparent resin layer of polycarbonate or the like isformed on a metal reflection film, and thereafter, the mediumidentification information 23 is recorded by irradiating, with a pulselaser (e.g., a YAG laser), the metal reflection film which lies at adepth of 0.1 mm from a surface of the optical disk. At this time, themetal reflection film seems to melt, and then, accumulate at bothboundary parts by surface tension. In this way, the metal reflectionfilm is partially removed, and thus, several reflection-film removedareas are formed. This creates a BCA area where medium identificationinformation is recorded which is used to identify the optical diskindividually.

Next, a method of recording the medium identification information in theBCA area of an optical disk will be described in detail. Herein, in thefollowing example, a method by which a record is made in the BCA area isdescribed, with respect to a metal reflection film which is made of anAg98Pd1Cu1 (wt %), or a metal reflection film which is made of anAl99Cr1 (wt %), as the metal reflection film. However, as long as thesame effect can be obtained, the present invention can also be appliedto other kinds of metal reflection films, a phase-change film, or anoptical magnetic recording film.

FIG. 8 is a block diagram, showing a configuration of amedium-identification-information recording apparatus which recordsmedium identification information in a BCA area. Themedium-identification-information recording apparatus shown in FIG. 8 isa BCA-pattern recording apparatus which is used to create a BCA area ina DVD-ROM. The apparatus includes: a motor 101; a rotation controlsection 102; an optical pickup 103; a laser drive section 104; awaveform setting section 105; a BCA-signal generation section 106; afocus control section 107; a pre-amplifier 108; and a system controlsection 109.

The rotation control section 102 controls rotation of the motor 101. Themotor 101 rotates an optical disk 100 at a predetermined rotationalspeed. The BCA-signal generation section 106 creates a BCA signal bymodulating medium identification information which is recorded in theoptical disk 100. Based on the BCA signal, the waveform setting section105 creates a laser modulation waveform. According to the lasermodulation waveform, the laser drive section 104 drives a high-powerlaser inside of the optical pickup 103. The optical pickup 103 convergesa beam of light emitted from the high-power laser, through its built-inoptical system, upon the optical disk 100. The pre-amplifier 108amplifies a reproduced signal which comes from the optical pickup 103,and then, outputs it to the focus control section 107. Using anamplified signal which comes from the pre-amplifier 108, the focuscontrol section 107 controls an objective lens inside of the opticalpickup 103, so that a beam of light can be converged on a metalreflection film of the optical disk 100. The system control section 109systematically controls operation of the rotation control section 102,the laser drive section 104, the waveform setting section 105, theBCA-signal generation section 106, and the focus control section 107.

Next, a recording operation will be described of themedium-identification-information recording apparatus which isconfigured as described above. First, based on an instruction from thesystem control section 109, the rotation control section 102 drives themotor 101 to rotate the optical disk 100. The laser drive section 104drives the high-power laser as a light source, and then, a beam of lightwhich is emitted from the high-power laser is applied to the opticaldisk 100 from the optical pickup 103. At this time, the focus controlsection 107 executes focus control so that a beam of light which hasbeen emitted from the high-power laser is converged on the metalreflection film of the optical disk 100.

Herein, reflected light from the optical disk 100 is detected by aphoto-detector inside of the optical pickup 103. Then, a reproducedsignal is outputted as an electric signal from the photo-detector. Thisreproduced signal is amplified through the pre-amplifier 108 and isinputted in the focus control section 107. In response to this amplifiedsignal, the focus control section 107 drives the objective lens of theoptical pickup 103 and moves it slightly in a focus direction on theoptical disk 100. Thus, it controls the optical pickup 103 so that thebeam of light can be converged on the metal reflection film of theoptical disk 100.

Next, the system control section 109 allows a position detector (notshown) to detect a position of the optical pickup 103 in a trackingdirection. Based on this detected positional information, the systemcontrol section 109 recognizes the optical pickup 103 to be located in asub-information recording starting position. Next, the system controlsection 109 instructs the BCA-signal generation section 106 to generatea BCA signal. Then, the BCA signal is outputted from the waveformsetting section 105, a BCA recording sequence starts, and the mediumidentification information is recorded in the BCA area.

In an optical disk where a 50 nm-thick metal reflection film made of anAgPdCu alloy was formed, using the above describedmedium-identification-information recording apparatus, an attempt torecord a BCA pattern (or a bar-code pattern) was made in a part whereneither a row of pits nor a guide groove was formed. However, even ifoutput power of a laser was heightened, a reflection-film removed areain which the metal reflection film was removed could not be created.

This is because a melting point of Al is 660° C. while a melting pointof Ag is 960° C. It takes a larger quantity of energy to melt the metalreflection film of an AgPdCu alloy. In addition, thermal conductivity ofAl is 237 W/(m.K) while thermal conductivity of Ag is 427° C. Therefore,a larger quantity of heat is diffused by conduction of heat, even thoughthe metal reflection film of an AgPdCu alloy is irradiated with a beamof light. Herein, in general, a melting point of metal is lowered bymixing a different metal. However, in order to secure an adequatereflectance ratio and avoid corrosion, a wt % of Ag in the metalreflection film cannot be reduced to at most 97%.

Next, in the optical disk where the 50 nm-thick metal reflection filmmade of an AgPdCu alloy was formed, a row of pits was formed at a trackpitch of 0.24 μm which was used in a BCA area of a DVD-ROM, and a BCApattern was recorded in that part. At this time, the BCA pattern couldnot be recorded at a predetermined width, and thus, information couldnot be reproduced. However, apart of this AgPdCu-alloy metal reflectionfilm melts, and a small reflection-film removed part could be formed.This is because a metal reflection film tends to be difficult to form onan inclined surface of an uneven substrate, and thus, a film thicknessof the metal reflection film in a pit inclined-surface part becomes thinlocally and heat conduction is hindered.

FIG. 9 is a sectional view of an optical disk in which a metalreflection film is formed on a substrate where pits are formed, and inaddition, a resin layer is formed on the metal reflection film. As shownin FIG. 9, metal reflection film 2 is formed on substrate 1 where pit 12is formed, and in addition, a resin layer 3 is formed on the metalreflection film 2. In this case, a film thickness of the metalreflection film 2 which is formed on an inclined-surface part 4 becomesthinner than the film thickness of the metal reflection film 2 which isformed on each of a pit-bottom part 5 and a flat-plate part 6. Thereby,a quantity of heat which is conducted around becomes smaller. Hence, thenarrower the track pitch of a row of pits becomes and the larger an areaof the inclined-surface part 4 becomes, the more easily heat will beconducted. Besides, in the inclined-surface part 4, a volume per unit ofthe metal reflection film 2 is smaller than any other part. Therefore, aheat capacity of the metal reflection film necessary for reaching themeting point becomes smaller, and thus, the meting point is reached witha lower irradiation power.

Based upon the above described knowledge, optical disks were prepared inwhich a 50 nm-thick metal reflection film made of an AgPdCu alloy wasformed on each substrate where a row of pits was formed at various trackpitches. Then, a BCA pattern was recorded in each optical disk. FIG. 10is a graphical representation, showing a measurement result of a defocusmargin of a BCA recording power which corresponds to a track pitch of arow of pits which is formed in an optical disk that includes a 50 nmmetal reflection film which is made of an AgPdCu alloy. The horizontalaxis in FIG. 10 is the track pitch (μm) of a row of pits and thevertical axis is the defocus margin (%).

As shown in FIG. 10, in an area where a row of pits was formed at atrack pitch of at most 0.54 μm, a BCA pattern could be recorded, andmedium identification information could be recorded. On the other hand,in an area where a row of pits was formed at a track pitch of at least0.54 μm, no defocus margin could be secured. Herein, a judgment that aBCA pattern was recorded, was made by setting and reproducing createdoptical disks in an assessment machine. The judgment was made based uponwhether or not medium identification information which was recorded inthe BCA area could be accurately reproduced. As the assessment machine,a reproducing apparatus was used in which a beam of light forreproduction had a wavelength λ of 405 nm and an objective lens had anumerical aperture NA of 0.85.

Herein, if one takes mass production of optical disks into account, onehas to consider a number of factors, such as dispersion of filmthickness of a metal reflection film, and a variation in a BCA recordingpower. Therefore, a defocus margin of at least 20% is required as itsadequate level. In FIG. 10, the track pitch at which a defocus margin ofat least 20% is obtained is at least 0.24 μm wide and at most 0.45 μmwide. Hence, if the track pitch of a row of pits which is recorded inthe BCA area is at least 0.24 μm wide and at most 0.45 μm wide, anadequate defocus margin can be secured, and medium identificationinformation can be recorded. A presumable reason for this is describedbelow.

Specifically, if the track pitch of a row of pits which is recorded inthe BCA area is beyond 0.45 μm, a number of pits per unit area becomessmaller, and thus, an area of inclined-surface parts of pits alsobecomes smaller. This hinders heat conduction from being cut offadequately. Therefore, if a heat capacity which is absorbed by a metalreflection film varies according to defocus, a BCA pattern whose noiseis low cannot be recorded.

On the other hand, if the track pitch is narrower than 0.24 μm, a pit istoo close to its adjacent pits. Therefore, formation of a land partbetween pits becomes inadequate, and an angle of an inclined-surfacepart of a pit becomes narrower. Thereby, a metal reflection film becomeseasier to form on inclined-surface parts of such pits, and thus, aneffect of cutting off heat conduction by formation of pits is reduced.Herein, in FIG. 10, a BCA pattern could be recorded and reproduced up toa point where the track pitch was 0.22 μm, while a BCA pattern could notbe recorded when the track pitch was narrower than 0.22 μm. Hence, inFIG. 10, a dotted line is an estimated line which corresponds to a trackpitch of at most 0.22 μm.

In addition, FIG. 10 shows that in an optical disk where a 50 nm-thickmetal reflection film made of an AgPdCu alloy was formed, a defocusmargin is dependent upon the track pitch. However, in an optical diskwhere a desirable jitter value was obtained, a metal reflection filmwhich was made of Ag or an Ag alloy might also have a film thickness ofat least 25 nm and at most 70 nm. In that case, if the track pitch of arow of pits which was recorded in a BCA area was at least 0.24 μm wideand at most 0.45 μm wide, a defocus margin could be obtained at the samelevel as described above.

Similarly, instead of a row of pits, the same experiment as describedabove was also conducted in an optical disk where a guide groove wasformed. Even in a case of a guide groove, a metal reflection film tendedto be difficult to form at an inclined-surface part of the guide groove,as was the case with a row of pits. Thus, if the track pitch of a guidegroove which was recorded in a BCA area was at least 0.24 μm wide and atmost 0.45 μm wide, a defocus margin could be obtained at the same levelas described above.

Therefore, in a case of an optical disk where a desirable jitter valuewas obtained, a metal reflection film which was made of Ag or an Agalloy had a film thickness of at least 25 nm and at most 70 nm, and ifthe track pitch of a row of pits or a guide groove which was recorded ina BCA part was at least 0.24 μm wide and at most 0.45 μm wide, then adefocus margin could be adequately secured.

Next, an optical disk will be described whose metal reflection film wascreated by using a metal reflection film which was made of an Al99Cr1(wt%) (hereinafter, referred to as an Al reflection film). First, anoptical disk was prepared where the Al reflection film having a filmthickness of 30 nm was formed. Using the above describedmedium-identification-information recording apparatus, an attempt torecord a BCA pattern was made in a part where neither a row of pits nora guide groove was formed. In this case, a part could be formed in whichthe Al reflection film was removed, and in addition, mediumidentification information which was recorded as the BCA pattern couldalso be reproduced. However, the Al reflection film was thinner than one(i.e., 50 to 70 nm) which was used in a DVD-ROM, and thus, an adequatedefocus margin could not be obtained. In addition, in the optical diskwhere the Al reflection film with a film thickness of 30 nm was formed,if a BCA pattern was recorded in an area where a row of pits was formedat a 0.74 μm track pitch which was used in the BCA area of the DVD-ROM,then the same result as described above was obtained.

Therefore, an optical disk was prepared where the Al reflection filmwith a film-thickness of 30 nm was formed on a substrate where a row ofpits was formed at various track pitches. Then, a BCA pattern wasrecorded. FIG. 11 is a graphical representation, showing a measurementresult of a defocus margin of a BCA recording power which corresponds tothe track pitch of a row of pits which is formed in an optical disk thatincludes an Al reflection film which has a film thickness of 30 nm. Thehorizontal axis of FIG. 11 is the track pitch (μm) of a row of pits, andthe vertical axis is the defocus margin (%).

Even in a case of the Al reflection film, in the same way as describedabove, a defocus margin of at least 20% is required at a time of BCArecording. In FIG. 11, a track pitch at which a defocus margin of atleast 20% is obtained is at least 0.24 μm wide and at most 0.45 μm wide.Hence, even in the optical disk where the Al reflection film with athinner film thickness than that of a DVD-ROM is formed, if the trackpitch of pits which is recorded in the BCA area is at least 0.24 μm wideand at most 0.45 μm wide, an adequate defocus margin can be secured, andmedium identification information can be recorded. A presumable reasonfor this is described below.

Specifically, if the track pitch of a row of pits which is recorded inthe BCA area is beyond 0.45 μm, heat capacity necessary for reaching amelting point becomes extremely small, because the Al reflection film isthin. Thereby, an edge part of a BCA pattern is not formed desirably,thus making louder noise of a BCA reproduction signal.

On the other hand, if a row of pits is formed at a track pitch of atmost 0.45 μm, the narrower the track pitch becomes, the more likely apit is formed at an edge of a BCA pattern. Thus, a melted Al reflectionfilm is kept from flowing at a part where a pit is formed. Hence, in anarea where pits are formed at a narrower track pitch, noise of a BCApattern becomes lower. As a result, if a row of pits is formed at atrack pitch of at most 0.45 μm, a BCA pattern which realizes an adequatedefocus margin can be recorded.

However, if the track pitch becomes narrower than 0.24 μm, an angle ofan inclined surface of a pit which is formed becomes narrower. Thisweakens a force which prevents the Al reflection film from flowing, andthus, an adequate defocus margin cannot be obtained.

Therefore, if a row of pits is formed on a substrate at a track pitch ofat least 0.24 μm and at most 0.45 μm, control of heat is easilyconducted even in an Al reflection film which has a thin film thickness.Consequently, the Al reflection film could be removed almost completely,and thus, a desirable BCA pattern could be recorded.

Herein, FIG. 11 shows that in an optical disk where a 30 nm-thick metalreflection film made of an Al99Cr1(wt %) was formed, the defocus marginis dependent upon the track pitch. However, in an optical disk where adesirable jitter value was obtained, a metal reflection film which wasmade of Al or an Al alloy might also have a film thickness of at least15 nm and at most 40 nm. In that case, if the track pitch of a row ofpits which was recorded in a BCA area was at least 0.24 μm wide and atmost 0.45 μm wide, a defocus margin could be obtained at the same levelas described above.

Similarly, instead of a row of pits, the same experiment as describedabove was also conducted in an optical disk where a guide groove wasformed. Even in a case of a guide groove, the same effect could beproduced. Thus, if the track pitch of a guide groove which was recordedin a BCA area was at least 0.24 μm wide and at most 0.45 μm wide, adefocus margin could be obtained at the same level.

Next, a multi-layer optical disk will be described which is amulti-layer optical recording medium which is formed by laminating aplurality of metal reflection films as information recording layers. Forexample, on a first polycarbonate substrate having a thickness of 1.1 mmwhere a row of pits is formed, a first metal reflection film which ismade of Al and has a film thickness of 45 nm is formed, using the abovedescribed magnetron sputtering apparatus. Onto this metal reflectionfilm, a second polycarbonate substrate having a thickness of 15 μm wherepits are formed is glued, so that its side where those pits are notformed comes into contact with the metal reflection film. As anadhesive, for example, a resin to be hardened by light or the like isused which is strong in terms of adhesive bonding. Then, on the secondpolycarbonate substrate which has been glued in such a way as describedabove, a metal reflection film is formed which is made of AgPdCu and hasa film thickness of 28 nm. On top of this metal reflection film, atransparent resin layer is glued which has a thickness of 70 μm. As anadhesive, for example, a pressure-sensitive adhesive sheet or the likeis used.

Even in a double-layer optical disk which was created in such a way asdescribed above, if the track pitch of a row of pits which was recordedin a BCA area is at least 0.24 μm wide and at most 0.45 μm wide, a focuswas adjusted at a time of a BCA recording, and thereby, a BCA patterncould be recorded in both layers. Hence, a defocus margin could beobtained at the same level as described above.

Herein, the method of creating a multi-layer optical disk is not limitedespecially to the above described example. Before a transparent resinlayer is glued, a plurality of substrates may also be formed, so that amulti-layer optical disk can be obtained. In this case, even if anoptical disk is layered, a focus is adjusted at a time of a BCArecording, and thereby, a BCA pattern can be recorded in a desiredlayer. In addition, when a transparent resin layer and a polycarbonatesubstrate are glued, a light-hardened resin and a pressure-sensitiveadhesive sheet are used. But instead of these, an adhesive andtransparent medium, such as a dry photo-polymer, may also be used. Or,without gluing a transparent resin layer, a transparent resin layer mayalso be formed by using only a pressure-sensitive adhesive sheet, oronly a light-hardened resin.

As described above, in this multi-layer optical disk, several layerswere glued, thereby heightening its recording density. In addition, thetrack pitch of a row of pits or a guide groove which was formed in theBCA area was set to at least 0.24 μm wide and at most 0.45 μm wide.Thereby, when a BCA pattern was recorded, a focus of a laser beam oflight was adjusted to metal reflection films in which the row of pits orthe guide groove was formed, so that a suitable laser power could beapplied. Consequently, a BCA pattern could be recorded which had lownoise and a desired width.

Herein, in a ROM optical disk, the shorter its recording time becomes,the lower its costs will be. Therefore, in each of the above describedexamples, it is desirable that a row of pits or a guide groove in theBCA area and a row of pits in the main-information area be formedsimultaneously. In addition, if the track pitch of a row of pits or aguide groove in the BCA area is largely different from the track pitchof a row of pits in the main-information area, when a master disk ismanufactured, a rotational speed of the disk has to be largely changeddiscontinuously. Or, the main-information area is adjacent to the BCAarea, and thus, the rotational speed of the disk needs to be controlledso that it becomes a desired rotational speed as fast as possible. Inorder to make its linear velocity constant, preferably, the track pitchof a row of pits in the main-information area should be equal to thetrack pitch of a row of pits or a guide groove in the BCA area.

INDUSTRIAL APPLICABILITY

As described hereinbefore, according to the present invention, by usinga beam of light for reproduction having a shorter wavelength and anoptical system having a higher numerical aperture, data can be recordedat a higher density than in a DVD ROM optical disk. In addition, eventhough thermal conductivity or a melting point, which is an intrinsicvalue of a metal reflection film, is different, by using a conventionalmedium-identification-information recording apparatus, mediumidentification information can be recorded so that an adequate defocusmargin can be secured. Hence, the present invention can be suitablyapplied to an optical recording medium, for example, an optical diskwhich has a circular-plate shape and is used to generate information, orthe like.

1-4. (canceled)
 5. An optical recording medium including a substrate anda reflection film formed on the substrate, and in which information isto be reproduced by irradiating the reflection film with a beam of lighthaving a wavelength of about 405 nm, the optical recording mediumcomprising: a main-information area where a row of pits is formed asmain data; a sub-information area in which medium identificationinformation is to be recorded in the reflection film partially asbar-code like patterns, wherein the medium identification information isto be used to identify the optical recording medium individually; and arow of pits or a guide groove formed on the substrate in thesub-information area, with a track pitch of the row of pits or the guidegroove being at least 0.24 μm wide and at most 0.45 μwide, wherein ashortest pit length of the pits formed in the main-information area isat least 0.12 μm wide.
 6. An information reproducing method forreproducing the optical recording medium according to claim 5, whereinthe reflection film is irradiated with a beam of light having awavelength of 405 nm, so as to reproduce information in themain-information area where the row of pits is formed, and thesub-information area in which the medium identification information isrecorded.